If a trade falls in the forest but doesn't generate revenue, does it make a sound?

Goldman Sachs Group President Gary Cohn insisted at a conference last week that his firm was expanding market share in fixed-income, currency and commodities trading. This is even though, as a portion of the aggregate trading revenue reported by big banks, Goldman's revenue has barely budged. Judged by revenue, Citigroup has increased its market share the most since 2011, according to a recent report from Credit Suisse Group. And the biggest gainers last quarter were Deutsche Bank and Morgan Stanley.

Mr. Cohn argued that since the current low-volatility environment depresses trading revenue, the normal way of gauging market share no longer works. Goldman's traders are winning more trades, he said, but "it's tough to see."

There is no direct way for investors to verify this. But it isn't implausible given the differing nature of banks' fixed-income businesses. Some have become flow monsters, focused on doing huge volumes of often lower-margin trades. Others, like Goldman, concentrate on areas that produce higher margins or would do so if volatility weren't so low.

Yet Mr. Cohn may be arguing a moot point—it is dependent on him being right that trading volumes and revenue will rise when markets "normalise." At that point, Goldman should reap the reward of a higher share of trades that produce higher margins. The risk is that rivals, even if they have given up on trades that don't make much, become far more aggressive if there is again money to be made.

In that case, today's market-share gains could end up being akin to a pig in a poke. Investors should insist on seeing what's in the bag before agreeing it is worth very much.